An inflatable airbag is employed as a safety device for an occupant of a vehicle. An airbag is inflated by gas generated by combustion of gas generating material. When the vehicle experiences a crash, a crash sensor sends a signal to an ignitor which ignites the gas generating material. The gas is directed into the airbag to cause inflating movement of the airbag into the vehicle occupant compartment for restraint of an occupant of the vehicle.
Ignition and combustion of the gas generating material generates gas at elevated levels of pressure and temperature. The gas carries vaporized, molten, and particulate products of combustion. The heat and the products of combustion could damage the airbag. A filter is therefore required to cool the gas and to remove the vaporized, molten and particulate products of combustion. However, the filter should not inhibit rapid inflating movement of the airbag into the vehicle occupant compartment.
U.S. Pat. No. 4,017,100 discloses a filter for the gas which is generated to inflate an airbag. The filter is located within a housing between gas generating material and ga exit openings in the housing. The filter includes layers of filter material arranged in three adjacent groups. Layers of the first group are screens which provide structural support for the layers of the other groups. Layers of the second group are formed of glass fibers, and serve as filtering layers. Layers of the third group are formed of steel wool, and also serve as filtering layers. An additional structural support screen is provided on each side of the third group of layers, and a perforated metal plate separates the filter layers from the gas generating material. The successive layers of filtering material decrease in porosity in the downstream direction of gas flow through the filter. The successively lesser values of porosity are designed to remove particles progressively from the gas as the gas moves in a substantially linear path through the filter.
Another type of gas filter for a vehicle occupant safety apparatus is shown in U.S. Pat. No. 3,972,545. The '545 patent discloses an elongated cylindrical housing in which gas flows from a gas generating material to gas exit openings through a plurality of filter layers. The filter layers include a pair of perforated plates which support a plastic bag containing PH neutralizing material, and axially alternating groups of fine and coarse wire screens. A third perforated plate separates the screens from the gas exit openings in the housing. The gas travels axially through the filter in a substantially linear path.
U.S. Pat. Nos. 4,017,100 and 3,972,545 disclose two types of filters used to filter gas for inflating an airbag. Other patents and the technical literature disclose a multitude of filter structures. Most, if not all, of the filter structures include wire screen, steel wool and/or ceramic material to filter the gas. Typically, combinations of these materials are used. In the fabrication of a filter structure and assembly of a filter structure with other parts, steel wool and ceramic filters can become damaged.